A transformer is a device that transfers electrical energy from a primary circuit to a secondary circuit by magnetic coupling. Typically, a transformer includes a primary winding coupled to the primary circuit and at least one secondary winding coupled to the secondary circuit. The windings are wrapped around a core of the transformer. An alternating voltage applied to the primary winding creates a time-varying magnetic flux in the core, which induces a voltage in the secondary windings. Varying the relative number of turns of the primary and secondary windings around the core determines the ratio of the input and output voltages of the transformer. For example, a transformer with a turn ratio of 2:1 (primary:secondary) has an input voltage (from the primary circuit) that is two times greater than its output voltage (to the secondary circuit).
Over-current protection devices are widely used to prevent damage to the primary and secondary circuits of transformers. For example, distribution transformers have conventionally been protected from fault currents by circuit breakers. Circuit breakers interrupt continuity in the electrical circuitry of the transformer upon detecting a fault therein. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset and reused multiple times.
It is well known in the art to cool high-power transformers and over-current protection devices thereof using a dielectric fluid, such as a highly-refined mineral oil. The dielectric fluid is stable at high temperatures and has excellent insulating properties for suppressing corona discharge and electric arcing in the transformer. For example, the dielectric fluid can suppress corona discharge and electric arcing that occurs when a circuit breaker interrupts the electrical circuitry of the transformer. Typically, the transformer includes a tank that is at least partially filled with the dielectric fluid. The dielectric fluid surrounds the transformer core and windings and at least part of the circuit breaker.
The dielectric fluid in the tank may recede for any of a variety of reasons. For example, the dielectric fluid may recede because of a leak in the transformer tank. It can be problematic and even dangerous if the dielectric fluid in the tank recedes below a particular level. For example, if the dielectric fluid recedes below one or more components of the circuit breaker, the dielectric fluid may not provide sufficient insulative protection during a fault condition. In addition, if the dielectric fluid has dropped below the level of an arc chamber in the circuit breaker, an arc produced on interruption will be in air medium and may not extinguish until major damage has been done to the transformer.
Therefore, it is desired to provide a circuit breaker that includes functionality for interrupting the electrical circuitry of the transformer when the dielectric fluid level of the transformer tank recedes to an unacceptable level.